1. Field of the Invention
This invention relates to a process and apparatus for reducing the emission of nitrogen oxides and carbon monoxide, particularly in furnaces or heaters fired with natural gas, wherein secondary air is introduced into a combustion chamber by forming a plurality of relatively high velocity secondary air jets that promote rapid mixing of a fuel and primary air mixture into the secondary air flow.
2. Description of Prior Art
U.S. Pat. No. 4,511,325 discloses a burner for reducing nitrogen oxides (NO.sub.x) emissions. Make-up air is introduced through delivery ports which are laterally spaced from a fuel injection nozzle and a main air opening. The make-up air is introduced through the air delivery ports at a sufficient pressure so that the air penetrates deep into the combustion chamber.
U.S. Pat. No. 4,601,625 teaches a gas burner having axially directed jet members positioned in an opening within a firewall. Fuel and air are premixed and then injected into a combustion chamber through tubular jets that extend into the combustion chamber. Secondary combustion air is introduced into the combustion chamber through secondary air jets that each have a discharge end directed at an angle to create swirling secondary air flow within the combustion chamber.
U.S. Pat. No. 4,629,413 discloses a burner which uses secondary air to reduce NO.sub.x emissions. Secondary air ports are positioned about the nozzle and are used to introduce secondary air into the combustion chamber.
U.S. Pat. No. 4,531,904 teaches fuel gas nozzles which inject fuel into combustion air flowing through an annular passage.
The combustion of fossil fuels results in the emission of pollutants to the environment, including oxides of nitrogen (NO and NO.sub.2) and carbon monoxide (CO). The exhaust gases of natural-gas-fired furnaces and heaters for residential or commercial applications typically contain NO.sub.x concentrations of 60-120 parts per million (dry, corrected to 3% O.sub.2) and CO concentrations of 5-20 ppm (dry, at 0% O.sub.2). It is desirable to reduce NO.sub.x emissions from furnaces and heaters at standard operating conditions without significantly increasing CO emissions.
The firing density of furnace heat exchangers can be limited by the point at which the CO emissions increase beyond acceptable levels. This is called the "sour point" of the furnace. For safety purposes, it is desirable to have a sour point at a firing density well above the operating point of the furnace. Alternatively, a combustion system that allows the sour point to be moved to higher firing rates, and therefore extend the operating range of the furnace, is desirable.